The present invention relates to a photoconductor for electrophotography, which is central to electrophotographic devices, including copiers, printers, and facsimiles etc. In particular, the present invention relates to photoconductors utilized in high speed (100 A4 size sheets per minute or faster), high resolution (dot densities of 300 dpi or more) electrophotographic applications.
In recent years, the focus of research and development in electrophotographic devices such as copiers, printers, and facsimiles, etc. has been on developing higher print speeds and higher resolutions. Conventional electrophotographic devices have a print speed ranging from 40 to 100 sheets per minute on A4 size paper. Additionally, electrophotographic devices that employ photoconductors made of amorphous selenium, particularly amorphous selenium-arsenic alloys, have dot densities of 240 dpi or less. Photoconductors made of amorphous selenium exhibit excellent resistance to printing fatigue.
Photoconductors made of amorphous selenium-arsenic alloys are manufactured by vacuum depositing the amorphous selenium-arsenic alloy onto a substrate with a surface roughnesses, Rmax., ranging from 0.8 to 1.2 .mu.m. Utilizing a cutting process, a photosensitive coating is produced that is between 60 to 80 .mu.m thick. The photosensitive coating is subjected to an aging treatment enabling the photosensitive coating to stand up to repeated exposures to both light and dark conditions for extended periods of time.
The image forming process for electrophotographic devices (hereinafter devices) employing a cylindrical photoconductor is shown in FIG. 2. While being rotated (shown by the circular arrow), the surface of the photoconductor is charged with electricity through a charging means 5. Next the photoconductor is exposed to light consistent with the image information through an exposing means 6. This produces an electrostatic latent image. The latent image is processed by a developing agent through a developing means 7 to form a patent image. The patent image on the surface of the photoconductor is transferred to a carrier sheet such as paper through a copying means 8. The image is fixed to a carrier sheet through a fixing means 9.
The photosensitive coating of a conventional photoconductor is subjected to comparatively high charging potentials of between 800 to 1200 volts. Because the conventional photosensitive coating is relatively thick (60 to 80 .mu.m), image defects such as point defects are prevented from manifesting themselves even when the substrate is relatively rough (Rmax.) of 0.8 to 1.2 .mu.m.
The problem with electrostatic latent image formation is that the higher the print speed, namely the larger the rotational velocity of the photoconductor, the less light is available to expose the surface of the photoconductor. Reduced light thus requires a more sensitive photoconductor. Also, the shorter interval of time between the exposing and developing processes in a photoconductor causes the developing process to begin before the surface potential has time to decay completely. Surface potential decay requires a period of time after the surface of the photoconductor is exposed to light. The short time available for decay leads to deteriorating image quality along with patent image disorders such as image contrast problems etc.
Referring now to FIG. 3, describes the charging, exposing, and potential decaying processes of a conventional photoconductor. Electrically charged surface potential decays as follows:
1) Mono-layered photosensitive coating 12 (which is, for example, positively charged thereby inducing a negative charge in substrate 1) is exposed to light; PA1 2) this exposure produces negative and positive carriers in mono-layered photosensitive coating 12; PA1 3) each carrier migrates towards the surface of substrate 1 or the surface of mono-layered photosensitive coating 12 depending on its charge; PA1 4) these carriers neutralize the electric charges on each surface to complete the decay of the electrically charged surface potential.
The migration time of the carriers determines the potential decay period or photoresponse. Low mobility of carriers and long potential decay periods cause conventional photoconductors to have poor photoresponses. This results in deterioration of image quality when applied to high speed devices.
It is possible to secure more time for potential decay by making the outer diameter of the cylindrical photoconductor larger, but there are limits to the size you can make the photoconductor. The size of the photoconductor is constrained by the size of the overall device. In order to improve resolution, photoconductors employ developing agents with very fine particles. This results in a higher dot density. However, because conventional photosensitive coatings are so thick, incident light causes the generated carriers to move transversely. This causes the images to blur and fade. An overall reduction in the sharpness of the images results.
On the other hand, reducing the thickness of the photosensitive coating poses practical problems. Thin photosensitive coatings cause white or black point defects to appear on the images. These defects are caused by burrs left on the substrate during the cutting process. The cutting process leaves the surface of the substrate with a roughness in the range of 0.8 to 1.2 .mu.m as measured on the basis of surface roughness termed Rmax. Additionally, conventional photoconductors lack the sensitivity required for high speed devices.